This invention relates to the art of producing thinwalled honeycomb structures, such as those that would have utility as a catalyst substrate or as a diesel particulate filter. Such substrates may be formed from extrudable material such as particulate ceramic and/or metal batches which may be sintered, and similar materials which have the property of being able to flow or plastically deform during extrusion, while being able to become sufficiently rigid immediately thereafter so as to maintain their structural integrity, in the manner set forth in U.S. Pat. Nos. 3,790,654 and 4,758,272. Alternatively, the honeycomb structure may be made of pleated thin porous sheets of filter material whose layers are interleaved with corrogated or crimped spacers with parallel corragations or crimps thereof extending substantially perpendicular to the folds of the pleated sheets, as disclosed in U.S. Pat. Nos. 2,884,091, 2,952,333 and 3,242,649.
The invention more particularly relates to improved method and apparatus for drying wet honeycomb structures, formed of such particulate material or sheets, of virtually any desired size and shape of transverse cross-section. Honeycomb ware is typically manufactured by extruding or fabricating ceramic material into logs, followed by the drying, cutting and firing of such ceramic logs. The drying of such honeycomb logs must be done very carefully in order to not induce stresses in the honeycomb ware pieces, produced by non-uniform drying and shrinkage, which can create distortion, warping or cracking. Conventional convection or oven drying cannot be used, particularly with the relatively large logs required for diesel particulate filters, which in their green state may be 16 to 18 inches long, transverse cross-sectional diameters of 6 to 16 inches, and weighing up to about 70 lbs., since by its nature, convection drying dries the ware from the outside inwardly, and the early outside drying results in the shrinking of the outer layers, thereby invariably leading to cracking or distortion, rendering the product unusable.
Some forms of dielectric drying through the use of dielectric heating have been utilized in the past. However, due to non-uniform radio frequency fields between the dielectric electrodes and the orientation of such electrodes relative to the honeycomb ware, the results obtained with such prior art devices and methods were not entirely satisfactory. Typically, the larger green ceramic honeycomb structures were initially subjected to dielectric drying for about 25 minutes followed by hot air convection drying for about 72 hours. Not only was the required total drying time excessive, but also the available radio frequency fields produced by the dielectric heating are not uniform, thus resulting in a variable drying pattern within the honeycomb structure.
Accordingly, it is the object of the present invention to provide improved method and apparatus for efficiently and uniformly drying green ceramic honeycomb structures without distorting, warping or cracking the structure and thus minimizing the development of harmful stresses within the structure.